The use of separately driven gear pumps for conveying a plastic medium from a mixing and processing apparatus is disclosed in DE 37 14 243 Al, DE 38 42 988 Al, and DE 39 17 523 C2.
In such gear pumps, the plastic medium to be conveyed is fed centrally, by means of a connection duct, to the suction side of the gear pump between the two rotating gears therein. The medium is pumped to the pressure side of the pump by the rotating gears through their engaged teeth and discharged through a delivery outlet.
In the production of highly viscous, shear-sensitive polymers by means of screw extruders, the use of a gear pump as a delivery means is conventional. Details concerning pressure buildup, control, and regulation of the independent drives of the pump and of the mixing and processing apparatus are disclosed in DE 38 33 777 C2. The gear pump is mounted with its flange in a radial connection with an intermediate channel at the outlet end of a single-shaft extruder. In all other known applications as a delivery pump, the suction side of the gear pump is placed in working cooperation with the mixing and processing apparatus by means of a channel of varying length.
A disadvantage of this arrangement is that deposits can build up in the channel between the mixing and processing apparatus and the gear pump. This occurs particularly during periods of shutdown and upon a change of material. A considerable time expenditure is necessary for cleaning purposes which results in considerable expense. In order to resist the high pressures in the extruding of plastic compounds, it is known from GB Patent 1,170,728 to arrange conveyer screws on both sides of extended shafts of the gears of a gear pump, which screws convey from outside to inside. In addition to the considerable expense for sealing the three resulting working spaces, there is also the basic disadvantage that the conveyor screws and the gear pump must always be operated at the same speed and thus, in practice, this arrangement is either impractical or can only be used for a very few materials.